8.8: Limits of Solubility

• When we dissolve a solid in some liquid, what is actually happening?– Take glucose added to

water• H-bonding between water

molecules and glucose molecules pulls the individual molecules into solution

• H-binding and Van der Waals interactions keep the glucose held together as a solid

Solubility: Saturated Solutions

When we keep adding solid to a liquid, the solid will dissolve until…

The solid is in equilibrium with the dissolved solute in a saturated solution

Solubility

The molar solubility, s, of a substance is it molar concentration in a saturated solution

A saturated solution is one in which the dissolved and undissolved solute are in equilibrium

8.9: Like Dissolves Like

• What is a good solvent to remove butter from your shirt? Soda from your carpet?

• We want to use a solvent that has similar chemical properties because of the “Like Dissolves Like” rule

Like Dissolves Like

• It all comes down to intermolecular forces…• When we dissolve something, we replace

some of the specific intermolecular forces in the solution with the same kind of interactions with the solute.

• Water (polar, hydrogen bonding solvent) will work well to dissolve polar or ionic solutes

• Fatty stains like gravy or butter can be removed with organic solvents like hexane or dichloroethene

Soap

• Soap is made by treating fats with strong hydroxide bases– In the resulting chemical reactions, the fatty acids

are knocked off and have a polar head and an acyl tail

• Soap molecules have hydrophobic parts (the tails) and hydrophilic parts (the head)

Summary: Solutions

1. For most ionic substances, dissolution increases the entropy of the system and they dissolve more readily at higher temperatures (since the free energy is dependent on TS)

2. For gas molecules, the entropy decreases when the temperature rises due to the contribution of Ssurr (solvent)

3. Dissolution depends on the balance between the change in entropy of the solution and the change in entropy of the surroundings.

Boiling Point Elevation and Freezing Point Depression

• The presence of a solute will increase the boiling point of the solvent due to the decrease in Gibbs Free Energy

This is one reason why people put salt into cooking water: It makes the temperature higher

However: In reality, this elevation is extremely low.

Freezing Point Depression

• The presence of the solute molecules in the solution decreases the freezing point of the solvent relative to its normal value

It does so because of the increased solution entropy caused by the presence of the solute.

Boiling Point Elevation and Freezing Point Depression

Freezing Point Depression = kf(molality)

Boiling Point Elevation = kb(molality)

8.17: Osmosis

• We’ve all heard of osmosis, but what is it really?

Osmosis is the flow of solvent through a semi-permeable membrane from a region of low solute concentration to one of high solute concentration

• In terms of entropy, this just makes sense, right?

Osmosis: Examples

Red Blood Cells ina) Normal Cellsb) Dilute Solutionc) Concentrated Solution

Osmosis in the Laboratory

Osmosis: The Science

• If we set up an apparatus with a semi-permeable membrane on one end, filled it with a sugar solution, and then placed it into pure water, what would happen?

Osmosis: The Science

• We can use this equation to determine the molar mass of a solute from osmotic pressure measurements

= iRTC

i = van’t Hoff i factor (determined experimentally and related to ion charge)

R = Gas constant

T = Temperature

C = Molar concentration